1. Field of the Invention
The present invention relates to touch input devices of the type used to enter data into a data processing system by intersecting at least one of a plurality of light beams with a finger or stylus, and more particularly to light emitters used in connection with such apparatus.
2. The Prior Art
A variety of types of touch input devices are known, and in one commerically available type of such units, a number of light beams is provided spaced slightly in front of a CRT so that at least one light beam is broken when a particular position on the face of the CRT is indicated as being touched with a finger or stylus. Each of the light beams begins with a light emitting source such as a LED, and a light responsive device such as a phototransistor is located at the other end of the beam. Interruption of the beam produces a signal at the output of the photoresponsive device associated with the interrupted beam, and this output can furnish time and position information with respect to the touch input.
Because, under ideal conditions, each light beam is initiated from one source and is terminated in one receptor, it is normally necessary to provide means for collimating the light along the desired path, and to prevent it from spreading to other photosensitive devices, to prevent the occurrence of interruption signals from photosensitive devices not directly associated with an interrupted beam. Typically the means for doing this has comprised aligned aperatures, or focusing lenses, or both. Other techniques involved gating the photoresponsive device into operation one at a time, so that at any given time only one LED and one phototransistor is operative, making collimation unnecessary. Still other techniques involved the gating of the photoresponsive in groups, the groups being so arranged that all of the devices but one in a group are relatively remote from the beam of the operating LED.
While the techniques of the prior art have met with considerable success, they introduce either complexity in terms of additional electronic circuitry, or relatively low efficiency in utilization of the light produced by the LEDs. For example, when a lens system is employed for collimating the light into a beam, only the light rays within the solid angle intersected by the operative portion of the lens is guided along the desired beam path. Light rays in other angles are scattered or absorbed, and do not form a useful contribution to the energy of the light beam. In addition, the lens reflects a portion of the incident light at each surface, and this reflected light is also lost.
In addition, when collimating or lens systems are used, extreme care is necessary in orienting the position and attitude of the parts. When a lens system is used incorporating lenses physically separate from the light emitters, they must be positioned accurately so that the light emitter is at the focal point of the converging lens. Also the lens must be accurately located coaxially with the desired path of the light beam. Even when an LED structure is used which incorporates a transparent housing with a curved surface approximating a converging lens, the unit must be located accurately relative to the desired path of the light beam.